1. Field of the Invention
The present invention relates generally to servo control circuits and, more particularly, to a servo control circuit for an image rotator used in an optical rotary head video recorder.
2. Description of the Background
There has been proposed apparatus for optically recording and/or reproducing video image data, known as a helical-scan optical recording and reproducing apparatus, and Japanese Patent Laid-Open Gazette No. 63-103440 describes an example of such helical-scan optical recording and/or reproducing apparatus. FIG. 1 is a schematic diagram showing an arrangement of the recording and reproducing apparatus of the type described in Japanese Patent Laid-Open Gazette No. 63-103440.
As shown in FIG. 1, a laser light source 1, a beam splitter 2, a mirror 3, and an image rotator 5, including a Dach prism 4, provide the light beam. The Dach prism 4 causes a laser light image to rotate around the optical axis thereof when it is rotated and, typically, image rotator 5 includes a motor (not shown) that rotates Dach prism 4.
The write laser light beam emitted from laser light source 1 travels through beam splitter 2, past mirror 3 to image rotator 5 and is incident on a beam splitter 7 arranged at an upper open portion of a rotary drum 6. Beam splitter 7 is normally located on a central axis of rotary drum 6, however, in order to make the drawing simpler in FIG. 1 the laser beam from laser light source 1 is shown being introduced to beam splitter 7 as though it became incident on beam splitter 7 from the lateral direction of rotary drum 6. Although rotary drum 6 is made of a transparent material, such as glass or the like, it may be formed as a metal cylinder similar to the rotary drum of a conventional video tape recorder (VTR). When the rotary drum is constructed as a metal cylinder, apertures 6a, 6b are bored through rotary drum 6 in the radial direction, whereby the write laser beam incident on the beam splitter 7 is passed through objective lenses 8a, 8b and apertures 6a, 6b onto a recording surface of an optical tape 9. In this way, a laser beam modulated by a video signal is written onto the recording surface of optical tape 9, which is made of an optical record medium such as magneto-optical film and is obliquely wrapped around rotary drum 6 for forming slant tracks along the length of the tape.
In FIG. 1, guide rollers 10a and 10b serve to wrap optical tape 9 around rotary drum 6 and the drum rotates counterclockwise so that tape 9 is transported in the direction shown by arrow A.
A detector 11 receives the laser beam that is reflected from optical tape 9 and that travels back through aperture 6a, beam splitter 7, image rotator 5, mirror 3, and beam splitter 2 and derives the video image data that was previously recorded on tape 9. Detector 11 derives a tracking error signal, a focusing error signal, and an RF (radio frequency) signal representing the video signal. The optical system within rotary drum 6, for example, objective lenses 8a, 8b and the like, is controlled by the tracking error signal and the focusing error signal supplied thereto as feedback signals.
In the aforenoted arrangement, image rotator 5 is servo-controlled to rotate at a rotational speed equal to 1/2 of the rotary angular velocity of rotary drum 6. This arrangement produces a video image.
In order to have image rotator 5 and rotary drum 6 servo-controlled so as to rotate in synchronism with each other, the rotation of rotary drum 6 is decelerated mechanically by using a gear or the like to rotate image rotator 5 at a rotational speed of 1/2 the angular velocity of rotary drum 6. Alternatively, as described in Japanese Patent Laid-Open Gazette No. 63-103440, a phase difference between a frequency-divided output signal, which results from dividing the rotation angular velocity signal generated from the image rotator 5 per revolution, is calculated to obtain a synchronization error signal and image rotator 5 is electrically servo-controlled by this synchronization error signal.
When the electrical control operation is performed, such that image rotator 5 is servo-controlled to rotate at a rotational speed equal to 1/2 the angular velocity of rotary drum 6 in synchronism with rotary drum 6 as described above, an electrical offset adjustment must be performed in order to correct a mounting error between pulse generators (not shown) that are mounted on the rotary drum 6 and image rotator 5. In this example of previously proposed apparatus, the synchronization error of angles between rotary drum 6 and image rotator 5 is detected based on a phase difference of respective pulses, that is, a time difference, so that the offset adjustment is equivalent to the adjustment for correcting the time phase difference. The true synchronization error angle is obtained by multiplying the time phase difference with the rotational velocity. Accordingly, when the rotational velocity changes, a predetermined offset adjustment provides a predetermined phase difference. There is then the substantial disadvantage that the amount of the synchronization error angle will be changed.
Further, because a synchronization error signal is obtained on the basis of a phase difference between one pulse per revolution and a 1/2 frequency-divided pulse, a servo control operation having high gain and high accuracy becomes impossible.
Furthermore, if the rotary drum and the image rotator are mechanically servo-controlled to rotate in synchronism with each other, such apparatus becomes large in size and, also in this case, a servo control operation of high accuracy cannot be performed with ease.